The Berwyn Slate Quarry is located on the east side of Maesyrychen Mountain, above the valley of the Eglwyseg river. It is a shallow, open hilltop working covering an extensive area, material taken by a short incline to a mill. Originally, finished product, maily slab, was dispatched by an incline down to the valley road, but later the Oernant tramroad ran through the mill area and so served the quarry.
The quarry is beleived to have been in use from the mid-nineteenth century. It is depicted in use as 'Clogau' on earlier editions of the Ordnance Survey County series maps of 1875 and 1899 (Denbigh. XXXIV.2). But by the time the third edition was published in 1912 it had fallen out of use. The quarry is associated with the nearby worker's housing at Cottage Row (NPRN 27007).
The quarry was re-opened at some point and appears to be still in use as the Berwyn Slate Quarry Ltd, Llangollen.

David Leighton & J.Wiles, RCAHMW, 14 January 2015